1. Field of the Invention
My invention pertains to apparatus for the reproduction of information, particularly video and/or audio information, with or without means for recording. More particularly my invention concerns such apparatus well adapted for selective retrieval of a plurality or multiplicity of information streams or items recorded successively on one and the same record medium, permitting ready access to any desired one of the information streams or items. The record medium may be in the form of a disc, magnetic tape, etc.
2. Description of the Prior Art
The video disc player, as of the Philips-MCA type, represents one of the typical applications of my invention. The principles of the video disc player apply also to the recording and playback of audio information by the pulse code modulation (PCM) scheme. The record disc for use with such video and/or audio disc players bears information in the form of a succession of short grooves or "pits" inscribed along a multiturn spiral track.
For reading the record disc the disc player usually has an optical pickup assembly comprising a light source such as a helium-neon (He-Ne) laser or semiconductor laser, an objective or collector lens for focusing the laser beam on the record disc, and a photodetector for translating into an electrical signal the information-modulated laser beam that has been reflected from the record disc. The rotation of the record disc about its own axis and the linear motion of the pickup assembly in the radial direction of the disc combine to enable the scanning laser beam to follow the spiral track on the disc.
Although the record disc may have a single stream of information recorded from the start to end of its track, it is also possible to record a plurality or multiplicity of streams or blocks of information one after the other along the track. In this latter case it will be convenient for the user if the disc player permits the playback of the information streams in a random, as well as serial, order.
One well known method for the selective retrieval of the information streams from the record disc utilizes a series of addresses also recorded on the disc, during the vertical retrace intervals of the composite picture signal to be reproduced. Let it be assumed that the operator wishes to retrieve, according to this conventional method, the third of the successive information streams on the record disc. He must know, first of all, the address of the starting point of the third information stream on the record disc. If the third information stream starts from address 100, for example, then the operator may input address 100 by means of the keyboard of the disc player system.
During the subsequent process of locating the third information stream on the record disc, the optical pickup assembly scans its track at normal speed, reading the successive addresses thereon and delivering an address-representing output signal to a comparator circuit. The other input to this comparator circuit is the keyboard-specified address 100. When the pickup assembly reads address 100 on the record disc, therefore, the comparator circuit produces an output either for suspending the relative scanning motion of the record disc and the pickup assembly or for starting the playback of the third information stream.
An objection to this prior art information retrieval system resides in the trouble involved in inputting the starting address of each desired information stream through the keyboard. As the number of digits making up the starting address of each information stream increases, the operator must depress the correspondingly increased number of keys.
In order to overcome the above objection it may be contemplated to provide a set of information selector switches corresponding to the respective information streams to be reproduced. The information retrieval system may be present so that upon actuation of any one information selector switch, the comparator circuit may receive a signal representing the starting address of the selected information stream.
This alternative is also objectionable, however, because the starting addresses of information streams may differ from disc to disc. For example, while the third information stream may start from address 100 on one record disc, it may start from address 200 on another. Thus a different set of the starting addresses of information streams must be preset on the information retrieval system for each record disc to be read.
The above problems hold with various other types of apparatus for the reproduction of information. One example is a magnetic tape record-playback device equipped to permit selective retrieval of information streams prerecorded one after the other along the length of magnetic tape.